Like Dan said, you may want to break the area down into sections. Length*width is for a rectangle, parallelogram, or square, but from that you can easily figure a triangle. A triangle is just half. Picture a business card with a line drawn from the top left corner down to the bottom right, that divides the rectangle into two triangles. And a circle is pi*r(2), but if you see a D shaped area you could get close by figuring it as one half of pi*r(2). And some things you can average out, like when you see an S shape edge. If you drew a line right down those curves it would look like $ with equal areas to the left and right.

When I have to calculate areas such as for an irrigation plan, I take several key measurements and with an engineers rule I draw an outline of the area to scale onto graph paper. One inch may equal ten, twenty feet, or whatever. Or, one square may equal that. If your drawing is fairly accurate, you can have the whole area scaled down into lots of little 100 or 400 s.f. squares which you can add up for your total.

Take a walking meter and run it along the curves. Multiply that times an average width. I find it works pretty well. I can get within 1/2 yard. Of course you want to make sure you aren't short 1/2 yard.

There is a tool called a planimeter. Draw the area to scale, trace over it with the planimeter, do a calculation, and done. I use it to find sqaure footage of irregular shaped pools and patios. Got mine on Ebay for 30$.